Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Polyethylene can be made in different processes in order to give different properties. Known families of polyethylene include high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and low density polyethylene made using high pressure reactors (LDPE). Within these broad classes many variations exist resulting from different types of polyolefin process technologies (for example, solution, slurry or gas phase) or from the use of different catalysts (for example, Ziegler-Natta or constrained geometry catalysts). The desired application requires a careful balance of rheological properties which will lead a person of skill in the art to select one type of polyethylene over another. In many applications, such as blow-molding and blown film applications, melt strength of the polyethylene is a key parameter.
The melt strength is a practical measurement that can predict material performance. In melt processing, good melt strength is important to maintain stability during processes such as coating, blown film production, fiber spinning and foamed parts.
Melt strength is related to several processing parameters such as bubble stability and therefore thickness variation during blown film production; parison formation during blow molding; sagging during profile extrusion; cell formation during foaming; more stable thickness distribution during sheet/film thermoforming.
This property can be enhanced by using resins with higher molecular weight, but such resins will generally require more robust equipment and more energy use because they tend to generate higher extrusion pressure during the extrusion process. Therefore, properties must be balanced to provide an acceptable combination of physical properties and processability.
The present invention generally pertains to ethylene/alpha-olefin interpolymer resins that can make films with improved optical properties (haze, gloss, and/or clarity), good mechanical properties (tear and puncture), and high shrinkage (shrink tension) which create high value especially for shrink films, such as collation shrink films. Additionally, a high modulus is advantageous. It is difficult to obtain a balance of these properties with one resin. For instance, high shrinkage may be obtained by a very high molecular weight. This high molecular weight, however, often results in poor optical properties as very high molecular weight materials tend to be highly elastic and upon extrusion create a high surface roughness on the film leading to lower optical properties. Another example is that generally to increase the modulus the density is increased. When the density is increased, however, the puncture properties generally decrease.
Additionally, this invention allows for the use of an existing polyethylene resin, which when reacted with an alkoxy amine derivative, is even more suitable for shrinkage films due to lower melt index (I2 or MI), higher melt strength, higher Viscosity Ratios, and higher melt index ratios (I10/I2).
The ethylene/alpha-olefin interpolymer of the present invention provides good properties (such as optical, tear, puncture, shrinkage, and modulus) without any one of such properties being unduly negatively impacted. The present invention is a new process for increasing the melt strength of polyethylene involving reacting molten polyethylene with an alkoxyamine derivative through regular extrusion processing. Accordingly, one aspect of the invention is a method for increasing the melt strength of a polyethylene resin comprising first selecting a polyethylene resin having a density, as determined according to ASTM D792, in the range of from 0.90 g/cm3 to 0.955 g/cm3, and a melt index, as determined according to ASTM D1238 (2.16 kg, 190° C.), in the range of from 0.01 g/10 min to 10 g/10 min and then reacting an alkoxy amine derivative with the polyethylene resin in an amount and under conditions sufficient to increase the melt strength of the polyethylene resin.
The present invention is a new process for increasing the melt strength of polyethylene involving reacting molten polyethylene with an alkoxyamine derivative through regular extrusion processing. Accordingly, one aspect of the invention is a method for increasing the melt strength of a polyethylene resin comprising first selecting a polyethylene resin having a density, as determined according to ASTM D792, in the range of from 0.90 g/cm3 to 0.955 g/cm3, and a melt index, as determined according to ASTM D1238 (2.16 kg, 190° C.), in the range of from 0.01 g/10 min to 10 g/10 min and then reacting an alkoxy amine derivative with the polyethylene resin in an amount and under conditions sufficient to increase the melt strength of the polyethylene resin.
The present invention may also increase the Viscosity Ratio of the resin, indicating good processability.